[sdiy] BBD question

[Ingo Debus]

Am 30.08.2008 um 23:36 schrieb Nicholas Gregorich:

> I guess this is the part I'm having a hard time with here, why is  
> there not aliasing between every single stage in a BBD?

Sampling creates copies of the baseband spectrum (you mentioned it  
yourself in anther posting), centered around multiples of the  
sampling frequency. If the anti-aliasing filter before the first  
stage does its job properly, there are no frequencies above half the  
sampling frequency present in the signal, so the sprectra do not  
overlap.

If this sampled signal is re-sampled (by the second and all following  
stages) with the same clock the copies of the baseband spectrum don't  
cause aliasing because they are folded back exactly into the baseband  
position (and into positions where already copies of the spectrum were).

Makes sense?

A different issue is, perhaps BBDs create other noises, not only the  
copies of the baseband spectrum. If these contain frequencies above  
half the sampling frequency then it would indeed make sense to place  
anti-aliasing filters between cascaded BBDs.

Ingo